The human brain is often said to hold 100 billion neurons. When researchers counted in 2009, adult male brains averaged about 86 billion.

A 2009 study found the adult male human brain contains on average about 86 billion neurons, not the often-quoted 100 billion.

Writing in the Journal of Comparative Neurology in 2009, F. Azevedo and colleagues noted that despite widespread quotes that the brain contains 100 billion neurons and ten times more glial cells, the absolute numbers remained unknown. They measured them using a method called the isotropic fractionator.

They found the adult male human brain contains on average 86.1 billion neurons and 84.6 billion nonneuronal cells. Only 19% of the neurons are in the cerebral cortex, even though it makes up 82% of total brain mass.